Direct fired space heating, where products of combustion are delivered to a space to be heated, is the most efficient method of heating the space. However, traces of incomplete combustion and combustion related pollutants can decrease indoor air quality. Indoor air quality concerns have resulted in the regulation of oxides of nitrogen dioxide (NO2) from fuel burning equipment where the products of combustion are directly vented into the heated space. This has given rise to a number of techniques and burner improvements to lower emissions. Fuel lean pre-mixing of the air and fuel, various air and fuel staging strategies, and flame protection from tempering air are all strategies that are employed by those skilled in the art of combustion.
Methods and apparatus have also been developed wherein several of these techniques have been employed together. For example, it is within prior art to utilize a fuel lean pre-mix burner in combination with flame protection to lower oxides of nitrogen and reduce carbon monoxide in direct fired air heaters applied to space heating. These systems require precise mixing of fuel and air in tightly controlled ratios and off-design performance can create more emissions than standard emissions equipment. Thus, there is further need for improved methods and apparatus that create low levels of NO2 and CO in direct fired space heating.